1. Field of the Invention
This invention relates to a novel zirconia film and a process for preparing it. More particularly, it is concerned with a zirconia film which is dense and improved in the thermal resistance, thermal insulation properties and corrosion resistance, and a process for preparing it.
2. Description of Prior Art
In general, zirconia films have excellent thermal resistance, thermal insulation properties and corrosion resistance, and are used as materials for protecting surfaces of metals or the like. In particular, zirconia in which Y.sub.2 O.sub.3, CaO or the like has been added is called "stabilized zirconia" or "partially stabilized zirconia", which is known to have the advantage that it is free from, or lightened of, the volume changes accompanied by phase transformations inherently occurring in zirconia at about 1,100.degree. C. and can be hardly broken for that reason. A film of this stabilized or partially stabilized zirconia has so high refractive index and so high oxygen ion conductivity that it is expected to be utilized in various uses as an optical functional film material, an oxygen sensor, a solid electrolyte for third-generation fuel cells and so forth.
Conventionally known processes by which such a zirconia film is prepared include chemical vapor deposition processes (which is hereinfter called "CVD processes"), as being relatively high in the film formation rate and being easy to handle. Among the CVD processes, noted is a process in which oxygen is used as an oxidizing agent as being advantageous in that it can be more industrial and have a high safety. This process is a process in which there are used zirconium halide as a zirconium source, oxygen gas as an oxidizing agent and, where the stabilized or partially stabilized zirconia film is prepared, a halide of yttrium or the like as a stabilizing agent, and these are previously mixed and introduced into a reaction vessel provided with a substrate kept at 900 to 1,400.degree. C. under atmospheric pressure to form a film on the substrate (G. Wehl et al., Proc. CVD-VII, 536 (1979)).
However, the zirconia film obtained by the above conventional process has the problem that it can have only insufficient thermal resistance, thermal insulation properties, corrosion resistance, etc. because of its low denseness.